Inverters are often used to convert DC power into AC power for one or more AC loads. In the case of polyphase inverters, it is often necessary to produce a neutral voltage which is also supplied to the loads. The neutral voltage can be obtained from the DC voltage provided to the inverter input, in which case the inverter is referred to as the half-bridge type. In a full-bridge type of inverter, the neutral voltage is derived from the phase output voltages.
A polyphase inverter includes a plurality of inverter legs, each of which comprises a pair of series-connected power switches in the form of transistors or thyristors which are connected across the conductors carrying the DC voltage. An inverter control develops pulse-width modulated (PWM) switching patterns which control the switches in each inverter leg. In a half-bridge type of inverter, switching patterns for the inverter switches are selected such that only harmonics which can be easily filtered are present in the inverter output. Generally, the inverter switching patterns are selected such that all harmonics up to the 2N+1th harmonic are cancelled, including triplen (i.e. odd multiple of three) harmonics, where N is the number of pulses per half-cycle produced by the inverter. Thus, in a three-phase half-bridge inverter producing seven pulses per half-cycle, all harmonics up to but not including the fifteenth harmonic are cancelled.
In the case of a full-bridge inverter providing balanced voltages and currents to a polyphase load, triplen harmonics appear not only in each of the phase outputs, but also in the neutral derived from the phase outputs. These triplen harmonics comprise common mode voltages which are in phase and have equal amplitudes. Such common mode voltages do not contribute to harmonic content in the output. However, if the inverter switching patterns for the inverter switches are different from phase to phase, triplen harmonics are produced across one or more phases of the load. These harmonics may increase the total harmonic content in the power provided to the load to an unacceptable level. Thus, as far as applicant is aware, there has been no attempt to individually regulate the output phase voltages of a full-bridge inverter by independently varying the switching patterns for the inverter legs.
An inverter control for a half-bridge inverter is disclosed in Parro II, U.S. Pat. No. 4,595,976, assigned to the assignee of the instant application. Base drive signals for controlling the switches of the inverter are obtained from switching patterns stored in a memory wherein the patterns are retrieved in accordance with one or more operating parameters of the inverter.
A further inverter control is disclosed in Woehrle et al., U.S. Pat. No. 4,290,108.